Alkene mechanistic insights

Fig. 17.19. cis-wc-Dihydroxy-lation of alkenes with catalytic amounts of Os(VIII)/stoichio-metric amounts of methylmor-pholin-/V-oxide [NMO] top) and with stoichiometric amounts of Os(VIII) (bottom), respectively, and pertinent mechanistic insights obtained thus far. 

Fig. 14.16. cis-vic Dihydroxylation of alkenes with Os(VIII) and pertinent mechanistic insights obtained thus far. 

Both in situ infrared and multinuclear NMR under less severe conditions have been used to gain mechanistic insights. For the hydroformylation of 3,3-dimethyl but-l-ene, the formation and hydrogenolysis of the acylrhodium species Rh(C()R)(C())4( R=CH2CH2Bur) can be clearly seen by IR. NMR spectroscopy has also been very useful in the characterization of species that are very similar to the proposed catalytic intermediates. We have already seen (Section 2.3.3, Fig. 2.7) NMR evidence for equilibrium between a rhodium alkyl and the corresponding hydrido-alkene complex. There are many other similar examples. Conversion of 5.3 to 5.4 is therefore well precedented. In the absence of dihydrogen allowing CO and alkene to react with 5.1, CO adducts of species like 5.6 can be seen by NMR. Structures 5.11 and 5.12 are two examples where the alkenes used are 1-octene and styrene, respectively. 

One paradoxical characteristic with the slow addition procedure is observed for the most hydrolytically resistant alkenes, where the reaction proceeds faster the slower the reagent is added. As can be deduced from the discussion above, the slow addition and the acetate effect are not mutually exclusive, and can be used in combination for the most arduous alkenes (Table 3, entries 2 and 6). With the mechanistic insight into the details of the asymmetric dihydroxylation, and the remedies to maintain the enantioselectivity in the addition step, the enanti-oselectivity in the chiral selectors (DHQD-CLB and DHQ-CLB) became limiting factors in the AD process. 

Antonova, N., Carbo, J., Kortz, U., et al. (2010). Mechanistic Insights into Alkene Epoxidation with H2O2 by Ti- and other TM-Containing Polyoxometalates Role of the Metal Nature and Coordination Enviromnent, J. Am. Chem. Soc., 132, pp. 7488-7497. 

In addition to the metal dihydrides and metal dihydrido substrate complexes, other mechanistically significant intermediates are sometimes detected. For instance, for Pd complexes, the intermediates formed upon incorporation of one hydride in the substrate (e.g., palladium vinyl intermediates) have been observed [56, 57). If the metal dihydride is observable in the presence of an alkene, further insight into the reaction mechanism can be provided using exchange-type pulse sequences to accomplish magnetization transfer from a hydride signal to the hydrogens of a product [27, 58). 

The photochemical addition reactions of amines to alkenes have provided a remarkably fertile area for synthetic and mechanistic investigations for over three decades. Each decade has brought the discovery of signihcant new reactions. Much of the body of work described in this review now appears to be complete, including the authors own work on stilbene-amine and styrene-amine reactions. Thus, as we pause to take stock of what has been accompHshed, we look forward to discoveries of new reactions and mechanistic insights. 

Whilst there are several examples of late transition-metal stabilised nitrenes in the literature (see below), very few experimental studies are available for copper-nitrene intermediates. These are proposed as important intermediates in copper-catal3 ed alkane amination and alkene aziridination reactions, therefore identification and characterisation is critical for providing new mechanistic insights. Ray et recently... 

The study of the stereochemical course of organic reactions often leads to detailed insight into reaction mechanisms. Mechanistic postulates ftequently make distinctive predictions about the stereochemical outcome of the reaction. Throughout the chapters dealing with specific types of reactions, consideration will be given to the stereochemistry of a reaction and its relationship to the reaction mechanism. As an example, the bromination of alkenes can be cited. A very simple mechanism for bromination is given below ... 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

Alkene chlorinations and brominations are very general reactions, and mechanistic study of these reactions provides additional insight into the electrophilic addition reactions of alkenes. Most of the studies have involved brominations, but chlorinations have also been examined. Much less detail is known about fluorination and iodination. The order of reactivity is F2 > CI2 > Br2 > I2. The differences between chlorination and bromination indicate the trends for all the halogens, but these differences are much more pronounced for fluorination and iodination. Fluorination is strongly exothermic and difficult to control, whereas for iodine the reaction is easily reversible. 

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